AB1510 INFLAMMATORY MYOSITIS – FINAL ANSWER?
Authors: M. Malik, E. E. P. Htut
Journal: Annals of the Rheumatic Diseases (2022)

Background Motor neurone disease may mimic inflammatory myositis. There is an overlap in clinical symptoms (such as muscle weakness, dysphagia and respiratory involvement) which may pose a diagnostic challenge. Careful history, thorough clinical examination and appropriate investigations can help to distinguish between the two. Monitoring the response to treatment is also crucial. We present a case that presented to rheumatology with progressive muscle weakness, she was investigated and treated for inflammatory myositis but was later diagnosed with motor neurone disease. Objectives To illustrate the need for thorough clinical assessment and investigations in patients with suspected inflammatory myositis and consider alternative diagnoses as clinical picture evolves. Methods A 68 year old lady, presented with 6 months history of progressive proximal muscle weakness. She also reported weakness of grip, affecting activities of daily life. She was finding it difficult to get up from chair. There was no swallowing difficulty, respiratory symptoms and skin rash. She had no weight loss or constitutional symptoms. She did not have any joint pain or features of connective tissue disease. She had history of hypertension. On clinical examination, she did not have any features of connective tissue disease. There was no evidence of joint synovitis or skin rash. Proximal power in upper and lower limbs noted to be 4/5 with intact reflexes. Initial investigations revealed mildly raised CPK at 287 with normal thyroid function and inflammatory markers. Immunological tests including ANA and myositis panel were requested alongside EMG, MRI thighs and CT chest/abdomen/pelvis. Myositis panel was positive for anti-MDA-5 antibodies (in the absence of any skin or respiratory symptoms). There was no evidence of malignancy on CT imaging. MRI thighs revealed fluid in the trochanteric bursae. She was trialled on oral prednisolone in view of her clinical features and positive anti-MDA5 antibody. However, there was no improvement in her symptoms or functional status. EMG revealed widespread neurogenic process affecting upper and lower limbs on both sides, without significant evidence of acute myopathy. The progressive nature of symptoms with the upper motor neurone signs raised the possibility of an evolving anterior horn cell disorder. She was referred to neurology and was subsequently diagnosed to have Motor Neurone Disease. The steroids were rapidly tapered off and stopped. Results Motor neurone disease is an important differential when considering inflammatory myopathy, particularly inclusion body myositis which causes more progressive muscle weakness. It is a neurological disorder characterised by degeneration of neurons. There is no clear cause identified. MND causes both upper and lower motor neurone signs. Symptoms include limb weakness, bulbar weakness that can affect speech and swallowing, muscle twitching and fasciculation. Respiratory weakness may occur. Thorough clinical examination and EMG findings help identify the diagnosis. Imaging may be needed. There is no cure. Management requires an MDT approach, primarily guided by neurology, including physiotherapy, occupational therapy, speech and language therapy and respiratory support. Conclusion This lady was initially treated for inflammatory myositis based on her symptoms, mildly raised CK and positive anti-MDA5 antibodies. On review of her clinical progress, we noted poor response to steroids and EMG results pointing at an alternative diagnosis of MND. This case highlights the importance of EMG in the assessment and management of patients with suspected inflammatory myositis as well as the need to closely monitor progress and re-evaluation where needed. Although she had anti-MDA-5 antibodies and mildly raised CK, she did not have any skin disease or other systemic features, inflammatory markers and muscle imaging was normal. Response to steroids was inadequate. References [1]Turner MR, Talbot K. Mimics and chameleons in motor neurone disease. Pract Neurol. 2013 [2]Ryan A, Nor AM, Costigan D, et al. Polymyositis Masquerading as Motor Neuron Disease. Arch Neurol. 2003 Disclosure of Interests None declared